FIG. 1 is a view showing a structure of a related-art RF cascode amplifier. The RF amplifier is a block which is made to minimize a noise while amplifying a minute signal, and is core to RF circuit technology.
The RF rectifier is a block which provides a power gain and should match input/output each other. Therefore, it is common that impedance matching inductors Lg and Ls are used as shown in FIG. 1.
In the case of a complementary metal oxide semiconductor (CMOS), a gate is formed of a pure capacitor. Therefore, it is difficult to match the CMOS with an antenna having real impedance. To solve this, Ls is inserted. The inserted Ls serves to provide real impedance as indicated by Equation 1 presented below:
                                          Z            in                    ⁡                      (            s            )                          =                              sL            g                    +                      sL            S                    +                      1                          sC                              gs                ⁢                                                                  ⁢                1                                              +                                    g                              m                ⁢                                                                  ⁢                1                                      ⁢                                          L                s                                            C                gs                                                                        Equation        ⁢                                  ⁢        1            
In addition, as shown in Equation 1, Lg is an inductor which is inserted for impedance matching.
A small signal equivalent circuit of FIG. 1 is illustrated in FIG. 2. As can be seen from FIG. 2, the RF cascode amplifier has a structure in which a current source trans-impedance amplified and converted at M1 is current-amplified at M2 again, such that two-stage amplification is performed and an amplification gain is maximized.
The RF cascode amplifier having the above-described structure ideally operates at a frequency of 10 GHz or less, but, in a high frequency band in which Cp, which is a parasitic capacitor, relatively increases, a current signal trans-impedance amplified and converted at M1 may be lost.
In addition, as shown in FIG. 3, the structure of the related-art RF cascode amplifier has a problem that a noise characteristic deteriorates as a frequency increases.id22=4KTγ2gdo2Δf  Equation 2
A gain obtained by the transistor M1 is expressed by Equation 3 presented blow:
                              A                      V            ⁢                                                  ⁢            1                          =                              Gm            ×            Load                    =                                                    gm                1                                            2                ⁢                                  ω                  0                                ⁢                                  C                                      gs                    ⁢                                                                                  ⁢                    1                                                  ⁢                                  R                  s                                                      ×                          1                                                                    ω                    0                                    ⁢                                      C                    p                                                  +                                  gm                  2                                                                                        Equation        ⁢                                  ⁢        3            
As shown in Equation 3, the gain may be divided into a Gm part and a Load part. Since the two parts are influenced by the parasitic capacitor, the gain of the amplifier rapidly decreases as the frequency (ω) of the signal increases, and as a result, the unique characteristic of the cascode may not be utilized.
However, in the related-art RF cascode amplifier, the parasitic capacitor Cp may not be minimized due to the sizes of defined transistors.